1. Field of the Invention
The present invention relates to a method for infusing and molding a liquid resin for producing a sandwich structure in a block, without performing pretreatment on a core material.
2. Description of Related Art
Currently, fiber reinforced resin compositions are widely used because they are light in weight and very strong. In particular, a filament reinforced thermosetting resin composition has been advantageously used in airplanes, ships, railroad vehicles, cars, golf clubs, tennis rackets, etc., as a substitute for a metal material because it is light in weight, has high rigidity, and is very strong. In addition, further reduction in weight can be anticipated because of the dynamics characteristic thereof, and hence a sandwich structure material is attractive.
However, in general, the sandwich structure material is produced by laminating a prepreg, then heating the resultant laminated prepreg in an autoclave under a pressurized condition, and hence the conventional method has a problem in that it necessitates a large amount of labor, thermal energy, and plant-and-equipment investment, thereby making manufacturing and processing cost high. Then, a method of pouring a liquid thermosetting resin into glass or carbon fiber textiles and then hardening it has been developed, which is called RTM (Resin Transfer Molding) and VARTM (Vacuum-Assisted RTM). In this method, an expensive prepreg is not used, the textiles are laminated simply, and an autoclave is not used, and hence production of a fiber reinforced resin composition can be performed at a cost lower than that of the prepreg method.
However, the above method of pouring the liquid resin, such as RTM and VARTM, is not suitable for producing the sandwich structure material using hollow cores of which the surfaces are not closed, such as a honeycomb core and an open cell foam core, because the liquid resin enters into the hollow cores to increase the core weight remarkably. Therefore, it is necessary to adhere the facing material to the core material which were fabricated separately according to the conventional method, to obtain a sandwich structure material. Recently, a method which includes heating and adhering a cover material to a core material beforehand to obtain a fabricated core material, installing the fabricated core material as well as the other members in a mold, clamping or bagging the mold, and thereafter pouring a liquid resin into the mold to mold has also been developed. However, this method requires a step for fabricating the core as an individual step, and moreover, this method has a problem in that when the molded object has a curved surface, it requires a metal mold for fabricating the core material to form the curved surface of the core material, in addition to the main mold for preparing the sandwich structure material, thereby increasing production cost.
In the Japanese Unexamined Patent Application, First Publication No. 2003-39579, the use of impermeable resin film and the non-low viscosity type film is disclosed, and in the Japanese Unexamined Patent Application, First Publication No H09-295362, the use of a specific resin and/or moisture cover film, and a film adhesive is disclosed. Moreover, in the Japanese Unexamined Patent Application, First Publication No. 2000-167950, application of a seal material which has thermosetting type adhesive performance is disclosed. However, although the producing process of these sandwich structures can be performed in batch processing in a metal mold, since it is necessary to heat it before pouring liquid resin therein, it is necessary to harden the adhesives and the seal material to seal the honeycomb core etc., and hence there is still a problem of the heating and hardening process being necessary to seal the core.
Thus, in the conventional method, heating is necessary to seal the hollow core of which the surface has not been closed, such as a honeycomb core and an open cell foam core. Particularly, in VARTM fabrication of a large article, a large oven is necessary, and the equipment expense and operating cost increase. Therefore, a method which can eliminate the heating process, and reduce consumption energy and labors is desired strongly.
Therefore, it is an object of the present invention to provide a process for producing a sandwich structure and an adhesive film used therefor, which can overcome the problem of the conventional technique. More specifically, it is an object of the present invention to provide a process for producing a sandwich structure and an adhesive film used therefor, which can produce a sandwich structure at a low cost, low energy consumption, and low labor, without heating and large scale equipment.